Much attention is focused on the development and testing of blood-stage malaria vaccines that target antigens expressed on the surface of merozoites (MSPs) given their functional importance in the invasion of erythrocytes and accessibility to serum antibodies. These efforts have been guided by in vitro studies of Plasmodium falciparum and in vivo studies of rodent malarial parasites that demonstrated the antibody- mediated inhibition of merozoite invasion of mature erythrocytes. However, data from studies of Plasmodium yoelii suggest that in vivo, these same antibodies are insufficient to prevent merozoites from invading reticulocytes. This does not appear to be due solely to differences in the receptors on host erythrocytes engaged by distinct merozoite proteins. This issue must be addressed if MSP-based vaccines are to be effective against Plasmodium vivax, a reticulocyte-restricted malaria parasite. Our hypothesis is that RBCs infected with reticulocyte-restricted malaria parasites localize to sites of erythropoiesis in vivo, mature and release merozoites into an environment that favors interaction with newly formed reticulocytes and reduces exposure to merozoite neutralizing antibodies. To test this hypothesis, we will conduct in vitro and vivo studies with P. yoelii blood-stage parasites that mimic the restricted or non-restricted host cell preferences of the human malarial parasites. We will focus on plasmodial proteins and variant surface antigens expressed on the reticulocyte surface, on their role in mediating adherence of pRBCs to vascular endothelium and on their role in immune evasion. Specifically, we will define the subset of plasmodial erythrocyte membrane protein genes expressed in P. yoelii infected reticulocytes which adhere to vascular endothelial cells and monitor changes in their expression in parasites under immune pressure. We will identify surface-exposed, P. yoelii reticulocyte membrane proteins that bind to receptors expressed on vascular endothelial cells. We will determine if antibodies that block adherence of P. yoelii infected reticulocytes to vascular endothelium in vitro also block localization of parasites to erythropoietic tissues in vivo and enhance the efficacy of MSP-based vaccines. These studies are built on the success of the malaria genome sequencing efforts, the ability to monitor the concurrent expression of large multigene families by microarray analysis, and the utilization of well-defined rodent models to conduct in vivo studies. Relevance: P. vivax causes 70-80 million cases of malaria each year. Efforts to develop vaccines to reduce P. vivax malaria are critical but must consider the unique aspects of the growth of this parasite in vivo. The proposed studies will increase our understanding of the factors that favor parasite invasion of subpopulations of erythrocytes in vivo to improve the design of vaccines that effectively inhibit the process.